1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor used for an electrophotographic image forming apparatus.
2. Description of Related Art
Inorganic and organic electrophotographic photoreceptors (hereinafter also simply referred to as “photoreceptors”) have been conventionally known as photoreceptors that are used for electrophotographic image forming apparatuses.
The term “electrophotographic” typically refers to an image forming process in which an image is formed by charging a photoconductive photoreceptor in the dark by means of, for example, corona discharge, then exposing it to light to dissipate the charges selectively in the exposed part so as to obtain an electrostatic latent image, and developing the latent image with a toner composed of a coloring agent such as dyes and pigments, a resin material and the like so as to visualize the image.
Compared to inorganic photoreceptors, organic photoreceptors are advantageous in flexibility in photosensitive wavelength range, ease of film forming, flexibility, film transparency, suitability for mass-production, toxicity, production cost and the like. Accordingly, organic photoreceptors are now used in most photoreceptors.
In recent years, organic photoreceptors with higher durability and higher image quality have been required.
For example, with the aim of achieving high abrasion resistance of a photoreceptor and forming an high-quality image, JP 2010-164646A proposes an photoreceptor in which an N-type semiconductor fine particles that have an electron transporting function and are made of aluminum oxide, titanium dioxide, tin oxide or the like are added to a cross-linked surface layer.
However, since such photoreceptors exhibit increasing residual potential after exposure when they are repeatedly used, they cannot stably form high-quality images for a long period of time. It is presumed this is because holes generated in a charge generating layer are trapped in the interface between a charge transporting layer and a surface layer and in the interface of particles of the surface layer due to the lack of a hole transporting function of the N-type semiconductor particles, and the holes cannot therefore effectively cancel negative charges on the photoreceptor surface.
Another photoreceptor known in the art is a photoreceptor in which an organic compound having a hole transporting function is added to a cross-linked surface layer. Photoreceptors of this type initially exhibit reduced residual potential, but the organic compound deteriorates and loses the function after repeated use, and the advantageous effect is eventually not exerted. Further, organic compounds having a hole transporting function generally have a plasticizing function, which reduces the film hardness of a surface layer.
Yet another photoreceptor known in the art is a photoreceptor in which P-type semiconductor particles are added to a cross-linked surface layer. However, it is difficult to reduce the residual potential to a sufficiently low level by using photoreceptors of this type because the hole transporting function is inferior to the electron transporting function (the hole mobility is lower than the electron mobility).